It’d be great if you have opportunities to impress potential PIs in interviews (either before or during the process), but whether they’re required or not depends on the school and the specific graduate program at that school that you’re applying to. Individual professors in popular graduate programs at large public universities, for example, generally don’t want you to contact them before you apply (or they’d be overwhelmed with such requests). Besides your research experiences (or publications if you have them), the single most important factor is, IMO, your recommendations (ideally from professors who’re known in the field, or at least to the PIs who’re evaluating your applications).
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